1. Field of the Invention
The present invention relates to a multistage centrifugal compressor having a tank which can be opened horizontally.
2. Description of the Related Art
The fundamental elements forming a multistage centrifugal compressor are a shaft equipped with a series of rotors, rotating round the machine axis, and a series of diffusers and/or diaphragms with return channels between the various stages, integral with a tank which contains the compressor.
Each rotor consists of a series of bladed disks; all rotors are assembled on the same shaft.
A diffusor follows each rotor disk.
Each diffusor is associated with a bladed return channel, which conveys the fluid to the subsequent rotor.
The whole set of each rotor together with the relative diffusers and return channels forms a stage, which is separated from the adjacent ones by annular diaphragms and labyrinth-seal systems to avoid recycling between one stage and the other.
The centrifugal compressors are equipped with diaphragms consisting of two half-diaphragms.
During the functioning of the multistage centrifugal compressor, the diaphragms are subjected to an axial force caused by the pressure differences due to compression of the fluid.
In order to counterbalance this force, it is therefore necessary to fix the half-diaphragms to the stator of the multistage centrifugal compressor.
In centrifugal compressors equipped with a tank which can be opened into two halves, a supporting ring is therefore envisaged for each single diaphragm, which is integral with the tank and is divided into an upper half-ring and a lower half-ring.
Each lower half-ring is welded to the lower, half-tank, and the corresponding upper half-ring is welded to the upper half-tank.
Each upper half-diaphragm is fixed to the corresponding upper half-ring, whereas each lower half-diaphragm is fixed to the corresponding lower half-ring.
This is due to the fact that the half-diaphragms undergo axial stress during the functioning of the compressor and, without the supporting rings, they would tend to move, causing, among other things, sealing problems among the various stages.
The assembly of the half-diaphragms in the tanks is extremely difficult, as it is necessary to centre all the half-diaphragms with the respective half-rings and also to centre the lower half-diaphragms with the corresponding upper half-diaphragms.
At the same time, it is extremely important to keep the seal between the various stages of the centrifugal compressor.
For this reason, the half-diaphragms are always fixed in advance to the corresponding half-rings.
In the assembly of a multistage centrifugal compressor, all the lower half-diaphragms are first inserted into the lower tank, followed by the shaft with the rotors.
Similarly, the upper half-diaphragms are inserted and fixed to the upper tank.
The upper tank can be assembled on the lower tank after being lifted by means of a bridge-crane, overturned by a rotation of 180° and correctly placed on the lower half-tank, in order to perfectly centre all the lower half-diaphragms.
High capacity/pressure multistage centrifugal compressors can be extremely heavy, up to 350 tons, and consequently the upper tank, with the upper half-diaphragms fixed, can weigh even 150-200 tons.
One of the disadvantages is that costly lifting systems are necessary, capable of lifting the total weight of the upper tank on which the upper half-rings have been welded and the upper half-diaphragms respectively fixed to the upper half-rings.
Another disadvantage is that it is not possible to effect controls on the positioning of the components inside the tank.
Furthermore, with respect to maintenance, a relatively common operation, such as the substitution of the labyrinth seal system, requires the overturning of the upper tank.
A further disadvantage is that in the case of particularly large and heavy machines, the overturning of the upper tank requires costly and complex equipment.